run your tires at the highest PSI you can stand (up to the max rated on the tire sidewall), make sure your alignment is correct, mix 1 oz. each of silver Power Service Diesel Kleen summer formula and Marvel Mystery Oil per gallon of diesel fuel, make sure your brakes aren't dragging, keep the speed down, draft off large semis to reduce wind drag-am I sounding like a big-time tightwad yet???

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1982 300DT 190K (Diesel Purge + synthetic oil=smoothness at last!!!)
2004 Ford E-350 6.0 L PSD 227K
2006 Dodge Ram 3500 SRW HO Cummins 4X4 48RE 42K (brute force tow vehicle)
2005 Scion xB wife's rolling pop can
1993 GMC Sierra C3500 6.2 142K